The present invention provides a novel enzyme and methods of using the enzyme for measuring glycine concentration. Specifically, the present invention provides an enzyme in which at least one amino acid residue is mutated so as to improve a property of a glycine oxidase which is associated with the measurement of glycine (e.g., activity of glycine oxidase for glycine, thermal stability of glycine oxidase, and substrate specificity of glycine oxidase for glycine,); and a method of analyzing glycine, that includes measuring glycine contained in a test sample using the modified enzyme; and the like.